Need help with 600v+ PSU

[black_labb]

i wouldnt personally put the 820ohm resistor in there unless you are happy to get some sag similar to a tube rectifier. when the valves are fitted the voltages will drop, and the 820ohm resistor will drop alot of voltage (it isnt now as there is no load though). if you want you can keep them in and see how you like it, but 820ohms seems way too high to me. remember ohms law. the voltage drop will be the current through the resistor multiplied by the resistance. for 200ma (rough idle estimate) you will drop ~160v. if you draw 400ma with more output, the resistor will be dropping 360v.

remember that the transformer will put out less voltage under load, and going a bit above the reccomended plate voltage wont cause a problem.

i'm just about to pick up my transformers at the post office

[andrew_k]

Good point black_labb, thankyou. The 820Ω were used for test fire-up and because they were the only >1W resistors I had on hand. I've laid it out so that those two resistors are very easy to change or remove entirely. I'll probably settle on something like 10Ω, if I leave them in at all.

I hope you're happy with the transformers, I certainly am. I wouldn't have known about Antek if it wasn't for you, so thanks again!

[black_labb]

anyone else notice what looks like a man in a blue cape sitting inside the power tubes in the last photo? his twin is in the other tube as well.
they matched those tubes for more than gain and current draw, they made sure the midgets inside them were the same too.

[andrew_k]

They're where mojo comes from.

[black_labb]

They're where mojo comes from.

and thats the difference between solidstate and valve equipment.

[Steve Conner]

According to none less than Horowitz and Hill, transistors have a little man inside too. You just can't see him because they're not made of glass.

http://books.google.co.uk/books?id=b...um=2&ct=result

[andrew_k]

Damn, and a two headed man at that!
That's progress for you.

[Suusi M]

this is quite similar to a circuit that I am playing with at the moment, 225 - 0 - 225
with a full wave bridge rectifier on the two 225 volt winding then grounding the negative, and using the center tap so I can use cheaper high voltage capacitors (47uF 450V)

its is a cross between an ampeg V4A's and a marshall power supply, in as only the B+ is used, not the center tap. The V4b uses a separate winding for the screens designed this way so it could use 6550A valves if the 7027A valve became unavailable.

Now from OK the data sheet, for 100 watts output from a pair of 6550A with a 600 volt psu 300 volt screen supply and grid bias of -32 volts the no signal current demand is 100ma and the full power current is 276ma. The max signal screen current is 33ma.

So the question boils down to how far will the B+ sag at full power IE with 276 mA from the B+?

Note that the lower winding will be loaded to 276 + 33 = 309mA due to the screen + anything else that you take off it. do not forget to multiply the DC current by 1.41 to get the AC current

[black_labb]

missed your post suusi. using the 2 halves of the voltage supply is a good idea. hv windings arent easy to come across.


i'm not sure about ac current being DC*1.41. i thought that was the way to find the peak voltage or peak current from the rms value. (i guess if you are talking peak ac current thats right, but thats not usually whats used). can someone confirm this?

[ecc83]

No, I don't think it will be a simple number like 1.414 either. It will depend upon transformer losses, ACTUAL cap' sizes and diode Vf's.

Best to put some loads on the thing, around 30-50% will do and work out what the PSU source resistance is ( dv/di) then you can predict up to the smoke point(!) how the thing will behave.

Dave.

[Steve Conner]

The average AC current is zero.

The rectified average AC current (what a cheap DMM reads when set to AC) is the same as the DC current.

The RMS AC current, which is probably what you want to know as it's what determines transformer and fuse ratings, is a complicated function of transformer winding resistance, leakage inductance, and filter cap size. It may well be about 1.5x the DC current, but in hi-fi amps with big toroidal transformers and oversized filter caps it can be considerably more than that.

[tedmich]

A standby switch would be nice, if you can find one capable of breaking 600V DC. I like to get the heaviest toggle switch I can find in DPST, and use the two poles in series to spread the arc out a bit. But even so, you're probably using the switch outside of its ratings.
.

these are always used WAY above their rating, and they carbonize and need replacement frequently, I found a nice environmentally unfriendly mercury wetted relay to handle ~700V that's very small that I am using

[unparalleled]

that explains why my old holden/wasp amps leave the plate voltage (700v) on during standby, and the high voltage isnt fused. i always wondered why. still dont understand why they use 2 fuses on the mains, a 3A on the live, and a 5A on the neutral. dont think that one will ever be explained.

The larger fuse was out of reach and sight in case the operator replaced the 3 Amp with a fuse larger than the proper size.
For instance if you had a valve with an internal short circuit the 3Amp will blow until the fault is remedied, if someone were to put in a 10Amp in the 3A holder the 5A fuse in the neutral line (hidden inside) would blow and prevent further goofing around until someone with a few more clues took it apart and fixed the problem.
The same thing could have been achieved with two fuses in series in the phase wire.

[black_labb]

The larger fuse was out of reach and sight in case the operator replaced the 3 Amp with a fuse larger than the proper size.
For instance if you had a valve with an internal short circuit the 3Amp will blow until the fault is remedied, if someone were to put in a 10Amp in the 3A holder the 5A fuse in the neutral line (hidden inside) would blow and prevent further goofing around until someone with a few more clues took it apart and fixed the problem.
The same thing could have been achieved with two fuses in series in the phase wire.

makes sense. good policy to keep the idiots from doing too many stupid things.

when i got my first wasp the internal fuse blew as i was just about to buy it. i checked the external fuse when i got home which was fine. i bought some new tubes thinking it was an output tube problem (didnt know much about amps at the time) and ended up seeing a photo of the fuse on the oz valve amp site, letting me know what the issue was, and letting me finish the purchase. guess its nice to have some spare tubes. wish the outside one blew first, but i guess fuses can get unreliable after a certain no. of years. the rest of the amp is still reliable though.